Hydrogen sulfide (H2S), a signaling molecule that elicits profound physiological effects, is a product of mammalian sulfur metabolism and is synthesized at relatively high rates. Since H2S is highly toxic, cells avoid its build-up by an efficient oxidation pathway that is housed in mitochondria and coupled to the energy-generating electron transport chain. The constituent proteins include sulfide-quinone oxidoreductase (SQR), a persulfide dioxygenase (ETHE1) and rhodanese in addition to the well-studied sulfite oxidase that catalyzes the conversion of sulfite to sulfate in the terminal step in the pathway. In red blood cells on the other hand, which exhibit robust H2S production capacity but lack mitochondria and hence the classical route for sulfide oxidation, the mechanism of sulfide clearance is unknown. In this study, we propose to address fundamental gaps in our understanding of the oxygen-dependent steps catalyzed by SQR and ETHE1, which combine to maintain low steady-state levels of H2S (in the 10-30 nM range in most tissues) and to elucidate the role of rhodanese in the sulfide oxidation pathway. We also propose to elucidate the potential role of methemoglobin in sulfide removal in red blood cells. These goals will be realized by addressing the following specific aims. (i) Using a combination of spectroscopic and kinetic methods we will elucidate the reaction mechanisms of human SQR and ETHE1. (ii) We will assess the relative catalytic efficiencies with which rhodanese generates glutathione persulfide, thiosulfate and H2S and elucidate the biochemical differences between two common polymorphic rhodanese variants. (iii) We will elucidate the mechanism of methemoglobin-dependent sulfide oxidation and assess its contribution to sulfide clearance at physiologically relevant concentrations of H2S and methemoglobin. Our studies will address fundamental questions such as how the sulfide oxidation pathway is wired, elucidate the reaction mechanisms of key enzymes that are potentially important pharmaceutical targets for modulating H2S levels and assess a novel and previously unexplored role for human hemoglobin in maintaining low plasma H2S concentrations.